Wireless communication systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450 and NMT-900, have long been in use successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-65 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communication services such as CDPD (Cellular Digital Packet Data).
Several types of access techniques are conventionally used to provide wireless services to subscribers. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communication channels wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. These bands are often reused in geographically separate cells in order to increase system capacity. Modem digital wireless systems utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to IS-95, IS-200, and Wideband Code Division Multiple Access (WCDMA) standards, achieve increased channel capacity by using “spread spectrum” techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code (i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communication systems operates).
Standard spread-spectrum CDMA communication systems commonly use “direct sequence” spread spectrum modulation. In direct sequence modulations, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communication medium (e.g., an air interface). The spreading code typically includes a sequence of “chips” occurring at a chip rate that normally is much higher than the bit rate of the data being transmitted. In a typical CDMA system, a data stream intended for a particular user (terminal) is first direct-sequence spread according to a user-specific spreading sequence. The resultant signal is then scrambled according to a cell-specific scrambling sequence. The spread and scrambled user data stream is then transmitted in a communications medium. Spread-spectrum signals for multiple users combine to form a composite signal in the communications medium. The channel estimation process has conventionally been accomplished by passing the received baseband signal on to a filter matched to the waveform of the pilot signal. By comparing the exact and filtered pilot signal, the channel random amplitude and phase can be estimated. The pilot signal may be a code-multiplexed pilot channel as the common channel used in IS-95, IS-2000 and WCDMA, or may be time-multiplexed pilot symbols used in some Traffic Channel configurations in WCDMA. The path time delay is assumed to be known. The desired pilot signals may be weak (for voice application) resulting in a bad channel estimate. In WCDMA, the channel parameters can also be estimated from the common pilot channel.
Downlink signals for different physical channels within a cell are transmitted from a base station in a synchronous fashion. The user-specific spreading codes are orthogonal, creating mutually orthogonal downlink signals at the transmitter. However, channel dispersion routinely results in a loss of orthogonality at the receiver, giving rise to intra-cell multi-user interference that can lead to degradation of receiver performance. In uplink signals, this interference can be intensified by the “near-far” problem (i.e., the higher contribution of energy from strong interfering signals intended for users located far from the base station than the signal intended for the desired user). Although the near-far problem can be alleviated by power control techniques on the uplink, power control does not solve the near-far problem on the downlink.
These problems may be exacerbated in “third generation” (3G) systems such as WCDMA systems. The 3G cellular mobile communication systems will support several kinds of communication services, including voice, images and even motion picture transmission. Therefore, the users will be transmitting their information signals using different data rates. Their performance requirements will vary from application to application. WCDMA with variable spreading factor (SF) and multicode modulation as a multirate scheme is emerging as one of the air interfaces for the 3G mobile communication systems. The high and different data rates and the large number of users, combined with multipath dispersive fading channels, cause severe inter-cell and intra-cell multi-user interference in both up and downlinks. This interference will limit the link capacity and/or degrades the quality of services. Moreover, the estimated wireless channel parameters will not be accurate because the pilot signal will be corrupted by the multiple access interference.
Previous work has demonstrated huge potential capacity and performance improvements as a result of using multi-user detection in spread-spectrum communications at the expense of increasing complexity of optimum structures. In general, a major problem with multi-user detectors and interference cancellers is the maintenance of simplicity. Most current detectors are designed for the uplink. For uplink interference cancellation, it is assumed that the receiver knows all the spreading codes. However, this assumption is not true for the downlink where the mobile unit only knows its own spreading codes. Furthermore, the interference cancellation algorithms proposed to date are very complex. For the downlink, since interference cancellation has to be performed at a hand-held battery-operated terminal, cost and power consumption are of great concern.
Most proposed techniques for interference cancellation are more suitable for uplink interference cancellation because the techniques are highly complex, requiring relatively high power consumption, and/or assume prior knowledge of the spreading sequence being used in the system. Therefore, there is a need for downlink interference cancellation techniques which minimize power consumption and do not require prior knowledge of the system spreading sequence.